1. Field of the Invention:
This invention generally relates to electric lamps and has particular reference to an improved incandescent lamp of the halogen-cycle type.
2. Description of the Prior Art:
Halogen-cycle incandescent lamps are per se well known in the art. A lamp of this type in which the regenerative-cycle is obtained by dosing the lamp with a small quantity of elemental iodine is described in U.S. Pat. No. 2,883,571 issued Apr. 21, 1959 to Fridrich et al. A method for making an iodine-cycle incandescent lamp in which the iodine is dosed into the envelope in the form of an iodide or hydroiodide of an element of Group IV of the Periodic Table is disclosed in U.S. Pat. No. 3,738,729 issued June 12, 1973 to Coxon et al.
The broad concept of dosing an incandescent lamp with a tin halide (such as SnI.sub.2, SnI.sub.4, SnBr.sub.2, SnBr.sub.4, SnCl.sub.2 and combinations thereof) to provide iodine, bromine or a mixture of iodine and bromine in combination with a vaporized tin halide in the fill gas is disclosed in Japanese Utility Design Publication No. 1971-23575 dated Aug. 13, 1971 (applied for by Toshiba Electric Company). The lamp is specifically designed to operate with a bulb wall temperature above 500.degree. C in order to surround the incandescent filament with vaporized tin halide so that the latter, by virtue of its high molecular weight, will inhibit the evaporation of tungsten from the filament.
A decorative type incandescent lamp that contains large amounts of elemental iodine (at least 15 milligrams per ml. of bulb volume) either alone or in combination with a vapor of various metallic iodides, including tin iodide and mercury iodide, and is operated in a vertical position to produce a luminescent pulsating colored flame effect is disclosed in U.S. Pat. No. 3,384,774 issued May 21, 1968 to J. F. English.
A high-efficiency type incandescent lamp that contains a tungsten filament, sufficient mercury to provide from 1 to 20 atmospheres of mercury vapor within the operating lamp, and elemental I.sub.2, Br.sub.2 or Cl.sub.2 along with at least one vaporizable metallic halide (including a tin halide) which is wholly or partially converted to the vapor state and is thus thermally excited to cause the metal to emit light emission in its characteristic atomic spectra is disclosed in U.S. Pat. No. 3,497,754 issued Feb. 24, 1970 to P. D. Johnson.
An incandescent lamp that contains a tantalum carbide filament and an atmosphere comprising hydrogen, vaporized carbon, an inert fill gas and one or two halogens (at least one of which is iodine or bromine obtained from a metal halide such as a halide of rubidium, cobalt, tin, silver, cadmium, aluminum, copper, zinc copper, mercury, nickel, and cerium) is disclosed in British Pat. No. 900,200 of Bryant et al.
Tungsten-filament lamps that are dosed with other types of halide compounds (such as HgI.sub.2, HgBr.sub.2 or mixtures thereof) are disclosed in Czechoslovakian Pat. No. 131,576 of J. Pavlata and in British Pat. Nos. 952,938 and 1,105,291 granted to J. Bowtell et al and J. Moore et al.
A bromine-cycle incandescent lamp that is dosed with a bromine-releasing compound such as hydrogen bromide is disclosed in U.S. Pat. No. 3,091,718 issued May 28, 1963 to J. Shurgan. An incandescent projection type lamp in which the halogen-cycle is obtained by dosing the envelope with predetermined amounts of mercury, elemental bromine and an inert fill gas such as nitrogen is described in German Pat. No. 1,928,442 of B. Kopelman.
Despite the various improvements made in halogen-cycle type incandescent lamps, it has been found that when tubular lamps which have been dosed with elemental iodine in the conventional manner and have long singly-coiled filaments and long design lives (over 1,000 hours nominal) are operated in an upright or slightly tilted position, premature blackening of the top portion of the lamp envelope occurs. The improvement disclosed and claimed in the aforementioned pending application Ser. No. 643,277 of A. D. Kulkarni avoids this problem by dosing the lamp with a carefully controlled amount of SnI.sub.4 which reduces the amount of vaporized elemental iodine within the operating lamp to a narrow range (from about 0.05 to 0.20 micromole per ml. of bulb volume). This increases the amount of free iodine that is in the atomic rather than molecular state during operation and permits the lamp to be burned in any position, including a vertical position, without premature bulb blackening.
While such SnI.sub.4 -dosed incandescent lamps operate satisfactorily in any burning position at the voltages for which they are designed and rated, it has been found that they develop deposits which discolor the lower portions of the lamp envelopes within a short period of time when they are operated in a non-horizontal burning position at voltages that are higher than their rated voltage. This discoloration problem is particularly acute in certain types of photographic equipment and the like which operate the lamps at approximately 112% of their rated voltage in a burning position 30.degree. from vertical in short "on-off" cycles.
Attempts to overcome this problem by dosing the lamps with controlled amounts of elemental iodine (or with various quantities of a compound such as methylene bromide which dissociates within the energized lamp to provide a bromine atmosphere) were not successful. Test lamps dosed with such additives developed deposits which discolored the top portions of the envelopes within short periods of time when the lamps were operated in an upright burning position under the aforementioned cycling and over-voltage conditions.
It would accordingly be very advantageous to provide an elongated halogen-cycle incandescent lamp which would not only permit the lamp to be operated in any position at its rated voltage without exhibiting premature discoloration but which would have a commercially-acceptable life span under extremely adverse operating conditions involving burning the lamp in a non-horizontal position at elevated voltages and in short "on-off" cycles.